What distinguishes the two different eruptive styles.  

Effusive Eruptions

- Produce mainly lava flows, low viscosity basaltic lava

- Mid-ocean Ridges & Oceanic Hotspots

-        

Explosive (pyroclastic) eruptions

- Produce clouds and avalanches of pyroclastic debris.  Occurs when gas expands in the rising magma but cant escape

- Subduction zones

The definition of viscosity.  

Viscosity – fluid’s resistance to flow

            -High viscosity – doesn’t flow easily (maple syrup in fridge)

            -Low viscosity – flows easily (heated maple syrup)

1.  Temperature UP, viscosity DOWN

2.  Silica content SiO2 UP, viscosity UP

The relative viscosity of the three main types of igneous magma.  

Basalt - LOW Si, LOW viscosity -flows

Andesite – between

The type of eruption produced by low viscosity magma and high viscosity magma. 

Basalt - LOW viscosity – effusive eruptions

Rhyolite - HIGH viscosity – explosive eruption

The difference between a pahoehoe flow, aa flow, and lava tube.  

Pahoehoe lava flow

-Basaltic lava that freezes into smooth, glassy, rope-like ridges

aa flow

-The surface layer of the lava freezes then breaks up due to the continued movement of lava underneath

-chunky

lava tube

The empty space left when a lava tunnel drains – when the surface of a lava flow solidifies while the inner part continues to flow

Hazards of lava flows. 

·      Basaltic lava from effusive eruptions is the greatest threat

·      Lava flows can bury roads, housing developments, and cars

·      Before the lava even touches it, the building may burst into flames from the intense heat

·      Forests, orchards and fields can burn and then be buried by rock and replaced by blackness

A couple ways to mitigate those hazards.  

-Danger assessment maps – map out the areas in the path of potential lava flows, lahars, debris flows, or pyroclastic flows

Evactuation – but exact times cannot be predicted

Diverting flows – using explosives, building dams, creating barriers etc…

Pumping Sea Water - May slow it down

What causes an eruption to be explosive?  

Effect of viscosity

-High-viscosity lava (andesitic and rhyolitic) lava can clog up a volcano’s plumbing causing a build up of pressure --

Effect of gas pressure

-The presence of gas in the magma increases the pressure because gas expands greatly as it rises toward the Earth’s surface.

-In basaltic magma (runny) – gas bubbles rise to surface and pop causing lava to fountain in sky

-In andesitic or rhyolitic magma (viscuous) – The gas bubbles can’t escape.  The gas bubbles expand and eventually shatter the partially solidified magma creating a pyroclastic eruption

The role of dissolved gasses in the magma. 

Effect of gas pressure

-The presence of gas in the magma increases the pressure because gas expands greatly as it rises toward the Earth’s surface.

-In basaltic magma (runny) – gas bubbles rise to surface and pop causing lava to fountain in sky

-In andesitic or rhyolitic magma (viscuous) – The gas bubbles can’t escape.  The gas bubbles expand and eventually shatter the partially solidified magma creating a pyroclastic eruption

Hazards related to stratovolcanoes, including ash fallout, pyroclastic flows, and lahars.  

Ash fallout :

-Lahars – Volcanic ash and debris mix with water, causing mudflows

Vesuvius 

-In 79AD

-Stratovolcano at a convergent boundary

-Pompeii buried in ash and pumice

Nevado Del Ruiz 

-Stratovolcano

-In 1985

-Columbia

-The volcano, which took 25,000, lives in mudflows, 13 November 1985

-Could have been prevented

Rhyolitic lava (most viscous, highest silica, coolest) when it erupts accumulates above the eruption vent forming a dome

– loose accumulation of pyroclastic grains